The present invention relates generally to electronic voltage regulation of field current in an alternator which, for example, can form part of a battery charging system for an automotive vehicle. More particularly, the invention is directed to a method and apparatus for using a pair of high voltage MOS Insulated Gate Bipolar Transistors (MOSIGBT's) to control the current in a field winding of such an alternator to both maintain a desired regulated output voltage and to protect the alternator and circuitry connected to the alternator from high voltage electrical transients generated if the alternator is disconnected from a load either intentionally or due to a fault condition. The present invention is particularly applicable to automotive battery charging systems and, accordingly will be described with reference to such a system.
In automotive battery charging systems, voltage regulation is commonly performed by monitoring an alternator output voltage/battery voltage and controlling the current through a field winding of the alternator to power automotive loads and to charge the battery. If a fault occurs within a charging system, such as the disconnection of the alternator from the battery, large transient voltages are generated at the alternator. The large transient voltages are due to the energy of self-inductance of the field winding and can damage the alternator, the regulator and/or other automotive loads if connected to the alternator output at the time the fault occurs.
In the prior art, attempts have been made to avoid damage by connecting voltage limiting and/or energy dissipating elements to the alternator output, the field winding of the alternator or the associated automotive loads. One example of a prior art attempt at transient control is illustrated in Murari et al, U.S. Pat. No. 4,455,585 wherein integrated circuits manufactured according to the usual processes for low voltage integrated circuit structures are employed. In Murari et al, a zener diode is connected either directly or switchably in series with a return diode to control a fault transient such that low voltage structures can be used. Alternately, the series connected zener diode and return diode are replaced by a low voltage PNP transistor which is connected across the field winding. A conventional regulator is connected to the PNP transistor base terminal to keep the transistor in the conducting state to return field current when a field current control transistor is inoperative and to produce a predetermined voltage between the collector and the emitter during a fault condition.
Unfortunately, Murari et al's low voltage PNP transistor cannot suppress the field decay transient to an effective level and may shunt the field winding during normal operation and hence contribute to loss of alternator regulation. Field winding shunting occurs due to operation of the low voltage PNP transistor in its inverse active region as the result of large reverse voltages which are repeatedly applied across the collector/emitter terminals of the PNP transistor as the regulation transistor is switched on and off.
Accordingly, there is a need for a method and apparatus for controlling the current in a field winding of an alternator to maintain a desired regulated alternator output voltage and to protect the alternator and equipment connected to an output of the alternator in the event the alternator is disconnected from a load it is driving, for example by a fault within a charging system utilizing the alternator, which otherwise causes potentially damaging high voltage transients at the alternator output.